Fuel injection pump for internal combustion engines

ABSTRACT

A fuel injection pump in which a relief channel is formed which communicates with the pump working chamber and is arranged to be blocked by a valve means, associated with a valve actuating means responsive to a pressure varying with engine r.p.m., during starting of the engine so that the fuel injection quantity is increased. Thus, improved starting characteristics of the engine can be obtained. Said relief channel may comprise a first channel communicating with the pump working chamber, and a second relief channel which has a portion thereof extending in the pump plunger to terminate in an outer periphery of the bottom end of said plunger and is so disposed as to communicate with the pump suction chamber after a predetermined delivery stroke length or prestroke has been executed. Communication between said first and second relief channels is adapted to be interrupted by said valve means at less than a predetermined engine r.p.m. As another advantageous arrangement, the relief channel may comprise a first relief channel which includes a port having one end communicating with the pump working chamber and the other end terminating in an outer periphery of the plunger and a communication channel having one end terminating in an inner periphery of the plunger barrel and the other end arranged for communication with a lower pressure zone, and a second relief channel allowing a restricted flow rate and communicating with the pump working chamber. The port and communication channel of said first relief channel are so disposed as to communicate with each other at the beginning of the delivery stroke of the plunger. Said first and second relief channels are arranged to be simultaneously blocked by said valve means during starting of the engine.

FIELD OF THE INVENTION

The present relates to a fuel injection pump for use with an internalcombustion engine, and more particularly to an improved distributor typefuel injection pump which can contribute to improvement in the startingcharacteristics of the engine.

In a fuel-injection type internal combustion engine including the Dieselengine, it is necessary to control the fuel injection quantity independency on the operating conditions such as load or engine r.p.m. Toimprove the starting characteristics of the engine, usually the fuelinjection quantity is so regulated as to be larger when starting theengine than in the ordinary or higher speed operation of the engine.

In a known distributor type fuel injection pump, the fuel injectionquantity is regulated by displacing the fuel quantity setting sleeveslidably fitted on the pump plunger arranged for pumping motion withinthe pump housing, by means of the centrifugal governor which is arrangedfor actuation in response to engine r.p.m. Thus, the position of theplunger can be varied at which the cut-off port leading to the fuelfeeding channel axially extending in the plunger becomes disengaged fromsaid fuel quantity setting sleeve, which determines the fuel injectionend. Said centrifugal governor is so arranged that a shifter beingdisplaced by the flyweights opening or closing in response to pumpr.p.m. causes the starting lever and the tension lever, both arrangedfor pivoting about a common fixed fulcrum, to be pivoted about saidfulcrum against the force of the starting spring and the governor springas the shifter is displaced, which in turn displaces said fuel quantitysetting sleeve which is connected to the common end of the levers.

However, according to such fuel injection quantity regulating mechanism,the displacement of the shifter caused by the opening of the flyweightscan have a limited stroke due to the spatial and structural limitations.Accordingly, it is difficult to set large the lift of the shifter. Also,there is a corresponding limitation in setting large the stroke throughwhich the starting lever can be pivoted, against the force of thestarting spring, from the starting position to the idling position wherethe starting lever comes into unison with the tension lever. For thesereasons, the injection quantity is insufficient during starting of theengine, which exerts an unfavorable influence upon the startingcharacteristics of the engine.

OBJECTS AND SUMMARY OF THE INVENTION

Therefore, the present invention has been devised in order to overcomemainly the above-mentioned drawback, and it is a primary object of theinvention to provide a fuel injection pump for internal combustionengines which can ensure an adequate amount of fuel injection quantityduring starting of the engine to improve the starting characteristics ofthe engine.

It is another object of the invention to provide a fuel injection pumpof the above type which can preclude the use of the starting spring andthe starting lever, and in which the tension lever is arranged to have alarger pivotal stroke so that the fuel injection quantity is regulatedwith higher accuracy during the ordinary or higher engine speedoperation when the governor is actuated.

It is still another object of the invention to provide a fuel injectionpump of the above type which has an increased injection quantity as wellas an injection advance during starting of the engine thereby to improvethe engine startability, and also has a reduced injection pressureduring an idling operation of the engine to obtain a reduced injectionrate thereby to suppress the combustion noise.

Said primary object of the invention can be achieved by the arrangementwhich includes a relief channel having one end thereof communicatingwith the pump working chamber and the other end arranged forcommunication with a zone under a lower pressure, a valve means providedacross said relief channel, and a valve actuating means communicatingwith a pressure source supplying a pressure varying with engine r.p.m.,wherein said valve means is actuated by said valve actuating meansresponsive to said varying pressure to block said relief channel duringstarting of the engine.

A first advantageous embodiment of the invention comprises a firstrelief channel having one end thereof communicating with the pumpworking chamber, a second relief channel having a portion thereofextending in the plunger to have one end thereof terminating in an outerperiphery of the bottom end of the plunger, said end of said secondrelief channel being so disposed as to disengage from an end edge of thefuel quantity setting member to communicate with the pump suctionchamber after a predetermined delivery stroke length or prestroke hasbeen executed, a valve means provided across said first and secondrelief channels for controlling communication therebetween, and a valveactuating means communicating with a pressure source supplying apressure varying with engine r.p.m., wherein said valve means isarranged to be actuated by said valve actuating means responsive to saidvarying pressure to interrupt communication between said first andsecond relief channels at less than a predetermined engine r.p.m.

A second advantageous embodiment of the invention comprises a firstrelief channel including a port having one end thereof communicatingwith the pump working chamber and the other end terminating in an outerperiphery of the plunger and a communication channel having one endthereof terminating in an inner periphery of the plunger barrel and theother end arranged for communication with a zone under a lower pressure,and a second relief channel allowing fuel to flow therethrough at arestricted rate, with one end thereof opening in the pump workingchamber and the other end communicating with a zone under a lowerpressure, a valve means provided across said first and second reliefchannels, and a valve actuating means arranged in communication with apressure source supplying a pressure varying with engine r.p.m., whereinsaid port and communication channel of the first relief channel are sodisposed as to communicate with each other at the beginning of thedelivery stroke of the plunger, and said valve means is arranged to beactuated by said valve actuating means responsive to the varyingpressure from said pressure source for blocking both said first andsecond relief channels during starting of the engine, opening both ofthem in a predetermined low engine speed range, and opening solely saidfirst relief channel in a higher engine speed range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partly longitudinal sectional view of a conventionaldistributor type fuel injection pump;

FIG. 2 is a partly longitudinal sectional view of a first embodiment ofthe distributor type fuel injection pump according to the invention;

FIG. 3 is a partly longitudinal sectional view of a second embodiment ofthe distributor type fuel injection pump according to the invention; and

FIG. 4 is a view showing on an enlarged scale an essential part of FIG.3.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a conventional distributor type fuel injection pump, inwhich fuel is aspirated from a fuel tank 1 by a fuel pump 2 driven by anoutput shaft of the engine, not shown, and fed under pressure into asuction chamber 4 formed within the housing 3 of the fuel injectionpump. In a known manner, a fuel pressure control valve 5 controls thepressure within the suction chamber 4 in dependence on engine r.p.m. sothat as the engine r.p.m. increases, so does the fuel pressure in thesuction chamber 4 in a predetermined manner.

A plunger 7 is slidably received within a plunger barrel 6 penetratingthe pump housing 3, for simultaneous reciprocating and rotating motionto perform the dual function of fuel pumping and distribution, ashereinafter described. More specifically, a cam plate 8 is providedintegral with the plunger 7 for rotation through a driving disk, notshown, by means of the drive shaft, not shown, of the fuel injectionpump driven by the engine. Said cam plate 8 has a cam surface formed, atequal intervals, with highs corresponding in number to the number of thecylinders of the engine. Said cam plate 8 has its cam surface urgedagainst rollers 9 retained on a roller holder, not shown, by a spring,not shown, so that the cam plate 8 and accordingly the plunger 7 iscaused by rotation of said drive shaft to simultaneously rotate andreciprocate.

During the suction stroke when the plunger 7 is moved downwardly in thedrawing, said pump working chamber 11 has an increasing volume. At thesame time, one longitudinal groove 12 of a plurality of such groovesformed in the peripheral surface of the top end of the plunger 7 facesone end of a supply line 10 formed in the housing 3 with the other endterminating in the suction chamber 4 so that fuel is sucked into thepump working chamber 11.

Now, when the plunger 7 is upwardly moved through the pressure ordelivery stroke, the communication between the supply line 10 and thelongitudinal grooves 12 is interrupted. Accordingly, the fuel introducedinto the working chamber 11 is delivered under pressure through achannel 14, which axially extends through the central portion of theplunger 7, and a longitudinal distributing groove 15, which is formed inthe outer periphery of the plunger 7, into outlet pressure lines 16penetrating the barrel 6 and the pump housing 3. The fuel then passesthrough delivery valve 17 into injection nozzles 18 by which it isinjected into the respective cylinders of the engine. Only one each ofthe outlet pressure lines 16, delivery valves 17, and injection nozzles18 is shown for the simplification of illustration. Said outlet pressurelines 16 are the same in number as the number of cylinders in the engineand are arranged circumferentially of the barrel 6 and the housing 3 atequal intervals so that fuel can be injected into each of the cylindersalternately in a predetermined order in accordance with thereciprocating and rotating motion of the plunger 7.

Also formed in a portion of the plunger 7 projecting into the suctionchamber 4 is a transverse cut-off port 19 which intersects the channel14 for communicating said channel 14 with the suction chamber 4. Saidcut-off port 19 is adapted to be obturated by the inner peripheralsurface of a fuel quantity setting sleeve 20 slidably fitted on saidportion of the plunger 7. Thus, when the plunger 7 is upwardly moved todisengage the cut-off port 19 from the upper end edge of the sleeve 20to cause the port to open into the suction chamber 4, fuel in thechannel 14 flows through the cut-off port 19 into the suction chamber14, and accordingly the delivery of fuel into the outlet pressure lines16 is interrupted thus to terminate the injection of delivery stroke ofthe plunger. Said fuel quantity setting sleeve 20 is arranged inengagement with levers 22 and 23 which in turn are arranged for pivotalmotion by means of an operating input mechanism for presetting a desiredengine speed and a governor mechanism for detecting an actual enginespeed (neither of them is shown) so that when the engine speed isdesired to be set to a lower value or when the engine is being operatedat a higher speed than a preset speed, said sleeve is displaced downwardin the drawing to obtain a sooner injection end during the deliverystroke, thus reducing the injection quantity.

Said two levers 22 and 23 are mounted in the housing 3 for pivotingabout a common fixed fulcrum 21. Said fuel quantity setting sleeve 20 isengaged by the starting lever 22 which is pivotable by the action of acentrifugal governor 24 having flyweights 24a, while said tension lever23 which is arranged on the pivoting side of the starting lever 22 andtensioned by a governor spring 25 engaged said starting lever 22 via astarting spring 26 having a smaller spring force.

Said governor spring 25 is connected with an end portion of an operatingshaft 28 via a connecting pin 27 in eccentricity therewith. Saidoperating shaft 28 has a portion thereof projecting outside the pumphousing 3 on which portion a control lever 29 is mounted. Said controllever 29 is connected with an accelerator pedal, not shown, so that thepretension of the governor spring 25 can be varied in response to theposition of the accelerator pedal. Thus, immediately after the enginehas been started when the centrifugal governor 24 still remainsunactuated, the fuel injection quantity is so regulated that anincreased amount of fuel is injected owing to the position of the fuelquantity setting sleeve 20 which is biased in the fuel increasingdirection by the starting spring 26. When the engine speed has risen upto a predetermined idling speed, the starting spring 26 becomescontracted so that the starting lever 22 is brought into unison with thetension lever 23 and pivots therewith until the tension of the governorspring 25 predetermined by the position of the control lever 29 comesinto equilibrium with the acting force of the centrifugal governor 24.The fuel quantity setting sleeve 20 is thus shifted so that the fuelinjection quantity has a suitable value for the load then applied to theengine.

However, as previously noted, such conventional arrangement inherentlyhas a limitation in the lift of the flyweights 24a of the centrifugalgovernor 24 which in turn imposes a limitation upon the amount ofdisplacement of the shifter 24b dependent upon the opening and closingmotion of the flyweights. Therefore, it is impossible to setsufficiently large the starting stroke which extends from the startingposition in which the starting spring 26 begins to be contracted to theidling position in which the starting lever 22 comes into unison withthe tension lever 23. Furthermore, as the starting lever 22 moves alongsaid stroke, the fuel injection quantity decreases linearly. For thesereasons, a sufficient fuel injection quantity is not obtained instarting, the engine thus providing an inferior engine startability.

The present invention has been devised in order to overcome such defectsin the conventional arrangement, and one embodiment thereof will now bedescribed with reference to FIG. 2. In FIG. 2, like reference numeralsdesignate like or corresponding parts or elements to those in FIG. 1.The plunger 7 has its interior formed with a fuel feeding channel 14aformed of a blind bore axially extending in a front end portion thereofto terminate in a top end surface thereof to communicate with a pumpworking chamber 11, and an axial cut-off channel 14b formed of anotherblind bore axially extending in a bottom end portion thereof tointeresect a cut-off port 19 radially terminating in an outer peripheryof the bottom end of the plunger. The channels 14a and 14b are separatedfrom each other by means of a central wall portion 7a of the plunger 7.They communicate respectively with annular peripheral grooves 32, 33formed in the outer periphery of the plunger 7 via respective transversebores 30, 31.

On the other hand, a fuel supply line 10 is formed in a top portion ofthe pump housing 3. Formed through the lateral wall of the plungerbarrel 6 in which the plunger 7 makes a reciprocal sliding motion is asuction bore 34 which communicates at one end thereof with said supplyline 10 via an annular groove 35 formed around the outer periphery ofthe barrel 6 and intersecting said supply line 10. Said suction bore 34is adapted to communicate at the other end with the pump working chamber11 defined by the housing 3, the barrel 6 and the plunger 7, during thesuction stroke of the plunger 7. Meanwhile, a valve bore 36 is formed inthe top portion of the housing 3 which is in communication with saidannular groove 35 via a small communication bore 45. A piston or valvebody 37 which has its interior formed with a channel 37a extending inH-shaped path is slidably received within said valve bore 36, to definea fluid or fuel chamber 38 between a top or left end face of the valvebody 37 and said valve bore 36. A spring seat or plug 40 is providedwhich has been threadedly fitted into an open end of said valve bore 36after a compression spring 39 was inserted thereinto, and urges anoutward or right end of the valve body 37 in an axial direction of thebore 36. Said plug 40 has its interior formed with a relief channel 41extending in T-shaped path. Also formed between the outward end of thepiston valve body 36 and the plug 40 is a chamber 42 with a variablevolume which may be supplied with a fluid or fuel pressure from upstreamof the fuel pump 2 through said T-shaped channel 41 and also through aspill tube 46 connected thereto, or may be open to the atmospherethrough said channel 41 and tube 46. A pair of fuel passages 43, 44extend from the pair of annular grooves 32, 33 formed in the outerperiphery of the plunger 7 through the barrel 6 and the pump housing 3to connect said annular grooves 32, 33 with said valve bore 36.

When the engine speed has exceeded a predetermined idling speed, anincreased fuel pressure within the suction chamber 4 causes acorresponding increase in the fuel pressure within the chamber 38through the supply line 10 and the annular groove 35, which in turncauses a displacement of the piston valve body 37 to bring an outwardend of said valve body 37 into contact with the plug 40. Then, theH-shaped channel 37a in the valve body 37 has two ports 37a' and 37a"thereof register with the respective associated open ends of the fuelpassages 43, 44, so that the fuel feeding channel 14a in the plunger 7communicates with the cut-off channel 14b through the fuel passages 43,44 and the H-shaped channel 37a within the piston 37. The H-shapedchannel 37a and the passages 43, 44 are so disposed that when the enginehas just been started and the fuel pressure within the chamber 38 hasnot yet increased with the piston valve body 37 kept off the plug 40, atleast one of the passages 43, 44 is obturated by the periphery of thevalve body 37 to interrupt the communication between the fuel feedingchannel 14a and the cut-off channel 14b.

On the other hand, the sliding sleeve or shifter 24b of the centrifugalgovernor 24 directly engages a tension lever 23 which in turn is engagedby a governor spring 25, to actuate the fuel quantity setting sleeve 20.Thus, the starting spring and the starting lever as conventionally usedcan be dispensed with.

With the above arrangement, as aforementioned, in the engine speed rangeof between the time when the engine was just started and the time whenthe engine nearly reaches the predetermined idling speed, during whichrange the engine has an increasing fuel injection quantity, the pistonvalve body 37 is kept at a distance from the plug 40 to keep interruptedthe communication between the fuel feeding channel 14a and the cut-offchannel 14b of the plunger 7. Accordingly, fuel feeding takes placethrough the fuel feeding channel 14a, the outlet pressure lines 16, thedelivery valves 17 and the injection nozzles 18 over the entire pressurestroke of the plunger 7 from the top dead point to the bottom dead pointirrespective of the position of the fuel quantity setting sleeve 20 thenheld, to carry out fuel injection in a sufficient amount into the enginecylinders, thus improving the starting characteristics of the engine.

Then, after the engine has reached a predetermined idling speed, thepiston valve body 37 is moved rightward against the force of spring 39into contact with the plug 40 owing to an increased fuel pressure withinthe fuel chamber 38, to establish a communication between the H-shapedchannel 37a and the fuel passages 43, 44 and accordingly that betweenthe fuel feeding channel 14a and the cut-off channel 14b as previouslymentioned. As a consequence, fuel is fed to the injection nozzles 18during the pressure or delivery stroke between the bottom dead point anda position of the plunger 7 at which the open end of the cut-off port 19disengages from the upper end edge of the fuel quantity setting sleeve20, in dependency on the position of the sleeve 20 determined by theequilibrium in force between the centrifugal governor 24 and thegovernor spring 25. Thus, an appropriate fuel injection quantity can beobtained which is in proportion to the load applied to the engine.

As explained above, according to the present embodiment, fuel injectioncan take place all the time during the entire pressure stroke of theplunger 7 after the engine has started until it reaches a predeterminedidling speed, thus enabling to obtain an adequate injection quantityrequired for starting the engine, thereby to achieve better startingcharacteristics of the engine.

Also, the present embodiment allows preclusion of the use of thestarting lever and the starting spring, which in turn rendersunnecessary adjustment of the stroke of the starting lever in assemblageof the fuel injection pump. Furthermore, spacewise and structurally thepivotal stroke of the tension lever which engages the governor springcan be set larger with ease owing to the removal of the starting leverand starting spring, which enables to achieve with a higher accuracycontrol of the fuel injection quantity during the ordinary or middle orhigh engine speed operation in which the governor is actuated, andbrings about other advantages.

Another embodiment of the invention will now be described with referenceto FIGS. 3 and 4. In FIGS. 3 and 4, like reference numerals designatecorresponding parts or elements to those in FIGS. 1 and 2.

Since the overall construction according to this second embodiment isgenerally similar or identical to that of the first embodimentpreviously described except for the points described hereinbelow,detailed description of the overall construction is omitted.

According to this embodiment, a communication channel 47 extends throughthe plunger barrel 6 and the housing 3, with one end thereof opening inthe pump working chamber 11 and the other end terminating in the innerperiphery of the cylinder bore 36. Also formed through the barrel 6 andthe housing 3 is a further communication channel 48 which has one endthereof terminating in the inner periphery of the barrel 6 which is insliding contact with the outer periphery of the plunger 7 and the otherend in the inner periphery of the bore 36. An annular groove 49 is alsoformed in the outer periphery of the plunger 7, which is so disposed asto register with an associated open end of said communication channel 48when the plunger 7 is at the bottom dead point, i.e., when thelongitudinal distributing groove 15 begins to face associated open endsof the outlet pressure lines 16. The annular groove 49 communicates,through a transverse bore 50, with the channel 14 which extends in theplunger 7 from the top end thereof to a point of the bottom end thereofwhere it intersects the cut-off port 19.

The outer periphery of the piston 37 is formed with two annular grooves51, 52 which are so disposed as to face associated open ends of saidcommunication channels 47, 48 at a predetermined stroke position of thepiston 37, in such a fashion that when the piston 37 is positionednearer to the plug 40 with respect to said predetermined position, theannular groove 52 with a larger width on the outward end side of thepiston 37 still engages with the associated open end of thecommunication channel 48, whereas the annular groove 51 with a smallerwidth on the top end side of the piston 37 is out of engagement with theassociated open end of the communication channel 47 (refer to FIG. 4).

One groove 51 of the annular grooves 51, 52 communicates with thechamber 42 through a passage 53 which includes a restriction portion 53aand extends through the piston 37 radially and axially thereof. Whilst,the other annular groove 52 communicates with the chamber 38 through apassage 54 extending in an L-shaped path through the piston 37.

With this arrangement, during starting of the engine, the pressure underwhich fuel is fed from the suction chamber 4 to the chamber 38 on thetop end side of the piston 37 through the chamber 35a is still low andaccordingly the piston 37 is biased more leftward than its illustratedposition by the force of compression spring 42, so that both the twoannular grooves 51, 52 of the piston 37 are kept obturated by the innerperiphery of the cylinder bore 36. Therefore, fuel is fed under ordinaryor usual pressure to the injection nozzles 18 through the longitudinaldistributing groove 15, the outlet pressure lines 16 and the deliveryvalves 17 to be injected into the combustion chambers of the engine allthe time from when the plunger 7 is at the bottom dead point until thecut-off port 19 gets disengaged from the upper edge of the fuel quantitysetting sleeve 20.

Next, in the low engine speed range mainly including the idlingoperation, the fuel feeding pressure increases to some extent such thata correspondingly increased pressure in the chamber 38 forces the piston37 to rightwardly move from said starting position into the illustratedposition, to allow the open ends of the communication channels 47, 48 toregister with the annular grooves 51, 52, respectively. Thus, at thebeginning of the delivery stroke of the plunger 7 when the plunger 7 isat the bottom dead point, the annular groove 49 engages with theassociated open end of the communication channel 48 as previouslymentioned, with the result that fuel is discharged from the annulargroove 49 into the suction chamber 4 which is under a lower pressure,through a first relief passage formed by the communication channel 48,the annular groove 52 and the passage 54 within the piston 37. Thus,fuel is prevented from being supplied to the injection nozzles 18 whichhave a larger flow resistance than said first relief passage. Then, upondisengagement of the annular groove 49 from the associated open end ofthe communication channel 48, the discharge of fuel is interrupted andfuel injection takes place with a time lag by an amount corresponding tothe stroke Sp (hereinafter called "prestroke") over which the annulargroove 49 and the communication port 48 overlap each other.Simultaneously, part of fuel within the pump working chamber 11 flows ata restricted rate into a lower pressure zone such as the suction side ofthe fuel pump 2 through a second relief passage consisting of thecommunication channel 47, the annular groove 51 and the passage 53, thusresulting in a limited increase in the fuel pressure in the pump workingchamber 11 which is confined within a certain range. This limitedincrease in the fuel pressure within the pump working chamber 11 leadsto a decrease in the fuel injection quantity per unit time or the fuelinjection rate. The decrease in the fuel injection quantity due to thelimited fuel pressure automatically causes the governor to act to makelonger the injection period so as to compensate for the decrease amountin the fuel injection quantity, thus enabling to reduce the combustionrate, which in turn achieves a reduction in the combustion noise. Thatis, conventionally the fuel injection was controlled by regulatingsolely the fuel injection quantity with the fuel injection rate keptconstant, which caused a shortened injection period in idling, with aresulting explosive combustion lasting for a very short time within thecylinders of the engine which led to an increase in the combustionnoise. The present invention can thus avoid such conventional drawback.

When the engine speed has further increased until the resultingincreased pressure within the chamber 38 forces the piston 37 to bedisplaced rightward of the illustrated position, the annular groove 51again comes to be obturated by the inner periphery of the cylinder bore36 whereas the other annular groove 52 still communicates with thecommunication channel 48, as previously noted. That is, in the middleand high engine speed ranges, flowing of fuel out of the pump workingchamber 11 through the communication channel 47 is thus interrupted sothat the working chamber 11 is kept under ordinary or usual pressure.Although, as in idling, also in these middle and high speed ranges, thebeginning of fuel injection during the pressure stroke of the plunger 7takes place with a time lag corresponding to the prestroke with respectto the stroke position of the plunger 7 as compared with the enginestarting, the actual fuel injection into the engine cylinders isadvanced with respect to the position of the piston within thecombustion chamber in compensation for an increase in the engine speedby means of a conventional timing control device, not shown, which maybe installed in the fuel injection pump.

Under said arrangement, provided that the injection quantity and theamount of injection advance for an idling operation are set so as tosatisfy the respective appropriate values, the fuel injection quantityis increased by an amount corresponding to the prestroke during startingof the engine as compared with that obtained by a conventional fuelinjection pump, thus resulting in an improved starting characteristic ofthe engine. At the same time, the fuel injection is carried out with atime advance by an amount corresponding to the prestroke Sp, whichresults in elimination of ignition lag within the engine cylinders whichoften occured during starting of the engine and is attributed to coolingof the engine such as in cold weather, thus further improving thestarting characteristics of the engine. In addition, according to theinvention, since as previously mentioned, the fuel injection rate isdecreased during idling of the engine which leads to a longer injectionperiod, the combustion noise can be reduced. Still further, in a higherengine speed operation, fuel injection can be performed under ordinaryor usual injection pressure and with an ordinary or usual injectionadvance characteristic by means of a conventional timing control deviceseparately provided in the fuel injection pump.

It is to be understood that the foregoing description relates topreferred embodiments of the invention and that various changes andmodifications may be made in the invention without departing from thespirit and scope thereof.

What is claimed is:
 1. A fuel injection pump for an internal combustion engine, which comprises: a housing; a suction chamber defined within said housing; a barrel mounted within said housing; a plunger mounted within said barrel for axial and rotary motion therein; a pump working chamber defined by the housing, the barrel and the plunger; outlet pressure lines for connecting said pump working chamber with associated injection nozzles; a first relief channel including a port having one end thereof communicating with said pump working chamber and the other end terminating in an outer periphery of said plunger and a communication channel having one end thereof terminating in an inner periphery of said barrel and the other end arranged for communication with a zone under a lower pressure; a second relief channel allowing fuel to flow therethrough at a restricted rate and having one end thereof opening in said pump working chamber and the other end arranged for communication with a zone under a lower pressure; a valve means provided across said first and second relief channels; and a valve actuating means arranged in communication with a pressure source supplying a pressure varying with engine r.p.m.; wherein said port and communication channel of said first relief channel are so disposed as to communicate with each other at the beginning of fuel delivery stroke of said plunger, and said valve means is arranged to be actuated by said valve actuating means responsive to a varying pressure from said pressure source for blocking both said first and second relief channels during starting of the engine, opening both of them in a predetermined low engine speed range, and opening solely said first relief channel in a higher engine speed.
 2. A fuel injection pump as recited in claim 1, in which said port of said first relief channel axially extends in said plunger from an end surface thereof facing said pump working chamber to communicate with a fuel injection quantity setting port radially opening in an outer periphery of a bottom end of the plunger.
 3. A fuel injection pump as recited in claim 1, in which said port of said first relief channel terminates in an outer periphery of said plunger through an annular groove formed therein, said annular groove being so located as to overlap said communication channel of said first relief channel over a predetermined length when the plunger is at a bottom dead point thereof.
 4. A fuel injection pump as recited in claim 1, in which said valve means comprises: a bore formed within said housing in communication on one side with said first and second relief channels and on the other side with at least one zone under a lower pressure; a piston slidably mounted within said bore; and a spring arranged for urging said piston in an axial direction of the bore against the force of said valve actuating means; wherein said piston has first and second passages extending therethrough to communicate with at least one zone under a lower pressure and so disposed as to communicate respectively with said first and second relief channels.
 5. A fuel injection pump as recited in claim 4, in which said second passage has a portion thereof formed as a restriction passage.
 6. A fuel injection pump as recited in claim 4, in which said first and second passages communicate respectively with the suction chamber within the pump housing and the suction side of an associated fuel pump.
 7. A fuel injection pump as recited in claim 4, in which said first and second passages terminate in an outer periphery of the piston through respective annular grooves with different widths, the widths and locations of said annular grooves being such that as the piston is displaced by said valve actuating means, neither of said first and second passages registers with said first and second relief channels during starting of the engine, both of them register with said relief channels in a predetermined low engine speed range, and only said first passage communicates with said first relief channel in a higher engine speed range.
 8. A fuel injection pump as recited in claim 4, in which said valve actuating means comprises a passage means having one end thereof communicating with one end of said bore and the other end with said pressure source supplying a pressure varying with engine r.p.m.
 9. A fuel injection pump as recited in claim 8, in which said passage means comprises a supply line formed within said housing with one end thereof communicating with said suction chamber which is supplied with a fuel supply pressure by a fuel pump arranged for rotation at a speed proportional to engine r.p.m. and the other end disposed for communication with said pump working chamber, and an annular groove formed in an outer periphery of said barrel with one side thereof intersecting with said supply line and the other side communicating with said bore. 